factors affecting microbial growth in food

The high positive Eh values registered by fruit juices (see Table 3.5) are largely a reflection of their low pH. Such a situation can occur in grain silos or in tanks in which concentrates and syrups are stored. The exact range of water activities allowing growth is influenced by other physicochemical and nutritional conditions but Figure 3.7 illustrates the range for a number of individual species of micro-organisms and Figure 3.8 demonstrates the interaction between temperature and water activity for Aspergillusflavus and Penicillium expansum. Cytoplasm is an aqueous solution and so must have a lower water activity than pure water; thus a micro-organism in an environment of pure water will experience a net flow of water molecules into the cytoplasm. Nitrogen needed for amino acids and nucleotides; some can synthesize … Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. When first introduced into brewing, hops probably contributed to microbiological stability, but this is less likely nowadays with the relatively low hopping rates used. Figure 3.9 shows the range of aw values associated with a number of different food commodities. 0.5 MPa) in a Gram-negative species. From the Nernst equation, it is clear that the hydrogen ion concentration will affect the Eh, and for every unit decrease in the pH the Eh increases by 58 mV. This group of bacteria, which includes such genera as Halo-bacterium and Halo-coccus, belong to the Archaebacteria and accumulate potassium chloride as their compatible solute. The acidity or alkalinity of an environment has a profound effect on the activity and stability of macromolecules such as enzymes, so it is not surprising that the growth and metabolism of micro-organisms are influenced by pH. 6.1.1. At this higher pH, the equilibrium shifts in favour of the dissociated molecule, so the acid ionizes producing protons which will tend to acidify the cytoplasm and break down the pH component of the proton motive force. As a consequence, herbs and spices may contribute to the microbiological stability of foods in which they are used. Hence the intrinsic factor of redox potential is inextricably linked with the extrinsic factor of storage atmosphere. Some of these changes, like those taking place during fermentation, are desirable, while others, like those resulting in food spoilage and food poisoning, are undesirable. Many lactic acid bacteria fall into this category; they can only generate energy by fermentation and lack both catalase and superoxide dismutase, but are able to grow in the presence of oxygen because they have a mechanism for destroying superoxide based on the accumulation of milli-molar concentrations of manganese. This is well illustrated by the food poisoning outbreaks in aerobic foods caused by the strict anaerobe When that same side cools down again the relative humidity increases and, although water molecules migrate back again, the temporary increase in relative humidity may be sufficient to cause local condensation onto the grain with a localized increase in aw sufficient to allow germination of fungal spores and subsequent spoilage of the grain. Redox dyes such as methylene blue or resazurin are sometimes used to indicate changes in Eh which are correlated with microbial levels. This partial dissociation of weak acids, such as acetic acid, plays an important part in their ability to inhibit microbial growth. Thus at 25 °C (298 °K) a water activity of 0.9 would correspond to a water potential of -143 atm or — 14.5 MPa. The pH of the food also significantly impacts the lethality of heat treatment of the food. If it cannot cope with this it will increase in size and burst. If the sample has a lower aw than the atmosphere then it will gain weight, if it has a higher aw then it will lose weight. At a water activity of 0.6, corresponding to a water potential of — 68 MPa, the cytoplasm would need to contain very high concentrations of an appropriate compatible solute and it is probable that macromolecules such as DNA would no longer function properly and active growth must cease. Ovotransferrin in egg white and lactoferrin in milk are proteins that scavenge iron from the medium. The fact that this is not observed in practice is, on reflection, hardly surprising since microbial growth results from the activity of a network of interacting and interrelating reactions and represents a far higher order of complexity than simple individual reactions. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. Microbial Growth There are a number of factors that affect the survival and growth of microorganisms in food. The measurement of water activity can thus be achieved by measuring the water content if the shape of the isotherm has been established. Considered as a hydride of oxygen (H2O) it has quite exceptional properties when compared with the hydrides of neighbouring elements in the periodic table such as ammonia (NH3), methane (CH4), hydrogen sulfide (H2S), and hydrofluoric acid (HF), see Table 3.8. With the exception of those soft drinks that contain phosphoric acid, most foods owe their acidity to the presence of weak organic acids. Bacillus subtilis (Eh — 100 to + 135 mV) produces rope in the open texture of bread and Acetobacter species growing on the surface of alcoholic beverages, oxidize ethanol to acetic acid to produce either spoilage or vinegar Obligate anaerobes tend only to grow at low or negative redox potentials and often require oxygen to be absent. This is achieved by the production of increasing concentrations of solutes which must not interfere with cytoplasmic function. Increasing the degree of unsaturation in a fatty acid decreases its melting point so that membranes containing higher levels of unsaturated fatty acid will remain fluid and hence functional at lower temperatures. In this article we will discuss about the extrinsic as well as intrinsic factors affecting the growth of micro-organisms in food. This is why they grow on foods with high moisture content such as chicken. Examples important in food microbiology are the lactobacilli and acetic acid bacteria with optima usually between pH 5.0 and 6.0. What are the characters Mendel selected for his experiments on pea plant? Factors Affecting. Decline phase 2.1. Both products contain the enzyme lysozyme which catalyses the hydrolysis of glycosidic linkages in peptidoglycan, the structural polymer responsible for the strength and rigidity of the bacterial cell wall. what the water activity is, unless the relationship between these two properties has been established. or even days 4. A graph showing the variation of growth rate with temperature illustrates several important features of this relationship (Figure 3.13). The temperature a food is stored, prepared and cooked at is crucial. Its presence and its influence on redox potential are important determinants of the microbial associations that develop and their rate of growth. With the notable exception of oxygen, most of the couples present in foods, e.g. It has a high E0 and is a powerful oxidizing agent; if sufficient air is present in a food, a high positive potential will result and most other redox couples present will, if allowed to equilibrate, be largely in the oxidized state. In this example the material has been allowed to equilibrate effectively at a known water activity before measuring the water content but Figure 3.11 demonstrates the differences which may be observed depending on whether a given water content is achieved by adding water to a dry commodity or removing it from a wet commodity. An example of a water sorption isotherm is shown in Figure 3.10. The decrease in Eh as a result of microbial activity is the basis of some long- established rapid tests applied to foods, particularly dairy products. Physical chemists would prefer to work with the chemical potential of water (µw), which is a complex parameter made up of a reference state, a water activity term, a pressure term and a gravitational term: which can be rearranged to give a new parameter, Ψ, known as the water potential having the same dimensions as pressure: For situations associated with everyday life on the surface of the Earth it is possible to ignore the pressure and gravity terms and a good approximation of the relationship between the water potential and water activity is given by Equation (3.24): where R (the gas constant )= 0.08205 dm3atm K-1 mol-1; and Vm (the molar volume of water) = 0.018 dm3 mol-1 ‘. Factors affecting microbial growth in food Processing factors • Processing factors: • Slicing • Washing • Packing • Irradiation • Pasteurization 43. Sofos, in Encyclopedia of Food Safety, 2014. The presence or absence of oxygen can naturally affect this, but for many anaerobes, oxygen exerts a specific toxic effect of its own. No single organism is capable of growth over the whole of this range; bacteria are normally limited to a temperature span of around 35 °C and moulds rather less, about 30 °C. This force is an electrochemical potential produced by the active translocation of protons from the cell interior to the external environment. Explain its significance. How the vascular cambium is responsible for secondary growth? Other contributory factors are thought to include changes in the physical properties of the plasma membrane adversely affecting solute transport; inhibition of key enzymes, particularly those involving carboxylation/decarboxylation reac­tions in which CO2 is a reactant; and reaction with protein amino groups causing changes in their properties and activity. availability to facilitate growth of microorganisms. In our everyday lives we think of water as existing in its liquid state between its freezing point (0°C) and boiling point (100 °C) and we might expect that this would limit the minimum and maximum temperatures at which growth could possibly occur. The factors influencing the growth of microorganisms are physical, chemical and biological in nature. It may take a very long time for the bulk of the commodity to increase in water activity, but condensation may occur on surfaces giving rise to localized regions of high water activity. particularly protein foods such as chicken and dairy products are rich in nutrients and moisture and so promote bacterial growth. When microorganisms grow in food, they cause varying degrees of change in the food's characteristics as a result of metabolic activity. View food spoilage.pdf from PUPLIC HEA HSC 1104 at University of Guyana. Antimicrobial components differ in their spectrum of activity and potency, they are present at varying concentrations in the natural product, and are frequently at levels too low to have any effect. Influence. As our foods are of plant and animal origin, it is worthwhile to consider those characteristics of plant and animal tissues that affect the growth of microorganisms. Another feature evident from Figure 3.13 is that the curve is asymmetric – growth declines more rapidly above the optimum temperature than below it. The reactions which take place in the cytoplasm do so in an aqueous environment and the cytoplasm is surrounded by a membrane which is generally permeable to water molecules which may pass freely from the cytoplasm to the environment and from the environment to the cytoplasm. Foods may contain multiple microenvironments. Humulones contained in the hop resin and isomers produced during processing, impart the characteristic bitterness of the product but have also been shown to possess activity against the common beer spoilage organisms, lactic acid bacteria. It is important to store, prepare and cook foods safely in order to reduce the risk of bacteria multiplying and causing foodborne illness. Carbon dioxide is not uniform in its effect on micro-organisms. The inability of an organism to utilize a major component of a food material will limit its growth and put it at a competitive disadvantage compared with those that can. The water content, however, may not give a good indication of how available that water is, i.e. Minimizing microbial growth on equipment, by cleaning and sanitizing, and in the product itself by adjusting storage temperature, pH, and other environmental factors. When S>>Ks, a micro-organism will grow at a rate approaching its maximum, but as S falls to values approaching Ks, so too will the growth rate. A class of antimicrobials known collectively as phytoalexins are produced by many plants in response to microbial invasion, for example the antifungal compound phaseollin produced in green beans. For example, pseudomonads, such as Pseudomonas fluorescens, which grows at an Eh of +100 to +500 mV, and other oxidative Gram-negative rods produce slime and off-odours at meat surfaces. A method known as the Landrock-Proctor method depends on gravimetrically measuring changes in water content of samples of the material after equilibration with atmospheres of known relative humidity which can be obtained using saturated solutions of a number of inorganic salts. Physical damage to the integument allows microbial invasion of the underlying nutrient-rich tissues and it is a commonplace empirical observation that damaged fruits and vegetables deteriorate more rapidly than entire products, and that this process is initiated at the site of injury. Start studying Factors affecting microbial growth in foods. Factors that. Obligate or strict aerobes are those organisms that are respiratory, generating most of their energy from oxidative phosphorylation using oxygen as the terminal electron acceptor in the process. glucose) for building blocks. This type of phenomenon can often account for localized caking of grain which had apparently been stored at a ‘safe’ water content. Psychrotrophs or facultative psychrophiles will grow down to the same low temperatures as strict psychrophiles but have higher optimum and maximum growth temperatures. Share Your PDF File This is known as the. Microbial growth •The microbial growth is the increase in number of cells rather than in size of individual cells. The growth of microorganisms in the body, in nature, or in the laboratory is greatly influenced by temperature pH, moisture content, available nutrients, and the characteristics of other organisms present. Answer Now and help others. To take account of this and the effect of other factors, it is more appropriate to define cardinal temperatures as ranges rather than single values (Table 3.11). parameters in the food to inhibit growth. Because of the different groups of organisms involved, it can also be different in character. Intrinsic and extrinsic factors play very important roles to maintain a microbiologically safe food system. Activity is proportional to concentration and the proportion­ality constant, the activity coefficient, approaches unity as the solution becomes more dilute. of cells dying 2. The factors are: 1. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. Bacteria need warmth to grow. This is analogous to buffering of a medium against pH changes and is, like buffering, a ‘capacity’ effect dependent on, and increasing with, the concentration of the couple. Considerable attention has been directed to the antimicrobial properties of those plants used as herbs and spices to flavour food (Table 3.6). Privacy Policy3. The relationship between water activity and water content is very sensitive to temperature and may seem to depend on whether water is being added or removed from a substrate. They obtain their energy through aerobic respiration. Spoilage of Food: 2 Factors | Food Microbiology, Food and Dairy Microbiology (Exam Questions) | Microbiology, Milk: Composition and Products | Microbiology. As redox conditions change there will be some resistance to change in a food’s redox potential, known as poising. Thus a compound like sodium chloride, which dissociates into two ions in solution, is more effective at reducing the water activity than a compound like sucrose on a mole-to-mole basis. As the temperature increases above the optimum, the growth rate declines much more sharply as a result of the irreversible denaturation of proteins and the thermal breakdown of the cell’s plasma membrane. The following points highlight the six main physical factors affecting the growth of microorganisms. Consequently they have a requirement for oxygen and a high Eh and will predominate at food surfaces exposed to air or where air is readily available. One factor often identified is the effect of CO2 on pH. These terms do not describe rigidly exclusive groups of micro-organisms but are useful in the context of studies of particular food commodities. Relative humidity and water activity are interrelated, thus relative humidity is essentially a measure of the water activity of the gas phase. This is because microbial inhibition by weak acids is not solely due to the creation of a high extracellular proton concentration, but is also directly related to the concentration of un-dissociated acid. . Similarly, exclusion of air as in modified vacuum packing or canning will reduce the Eh. Oxygen, which is present in the air at a level of around 21%, is usually the most influential redox couple in food systems. Factors affecting microbial growth in food • Intrinsic Factors • Environmental Factors • Implicit Factors • Processing Factors Also, like buffering, poising is greatest when the two components of a redox couple are present in equal amounts. Microbial growth on meat products, as well as other foods, is affected, not only by the type and level of initial contamination but also by various factors associated with the product (intrinsic) or its environment (extrinsic). As measured with the glass electrode, pH is equal to the negative logarithm of the hydrogen ion activity. All foods were at “some stage part of living organisms and, as such, have been equipped through the course of evolution with ways in which potentially damaging microbial infections might be prevented or at least limited. Vinegar and lemon juice have a similar effect. Anaerobic metabolism gives the organism a lower yield of utilizable energy than aerobic respiration, so a reducing environment that minimizes the loss of valuable reducing power from the microbial cell is favoured. At temperatures above the maximum for growth, these changes are sufficient to kill the organism – the rate at which this occurs increasing with increasing temperature. However, it is important to note that, even if active growth is impossible, survival may still occur and many micro-organisms can survive at very low water activities and are frequently stored in culture collections in this form. Water activity can be measured by measuring the equilibrium relative humidity of the atmosphere in contact with the sample. An oxidation-reduction (redox) reaction occurs as the result of a transfer of electrons between atoms or molecules. Some micro-organisms are killed by prolonged exposure to CO2 but usually its effect is bacteriostatic. The tendency of an atom or molecule to accept or donate electrons is expressed as its standard redox potential, E0‘. Able to grow on dry foods. Bacteria need moisture in order to grow. By carrying out measurements of weight change over a range of relative humidity it is possible to extrapolate to the relative humidity which would cause no weight loss and thus corresponds to the aw of the sample. Thus: where (aH) is the hydrogen ion activity and [H+] the hydrogen ion concentration. : It should be noted that water activity is a colligative property, that is to say it depends on the number of molecules or ions present in solution, rather than their size. It is usually composed of macromolecules relatively resistant to degradation and provides an inhospitable environment for micro-organisms by having a low water activity, a shortage of readily available nutrients and, often, antimicrobial compounds such as short chain fatty acids (on animal skin) or essential oils (on plant surfaces). and minerals 3. In fruits, however, a lower pH prevents bacterial growth and spoilage is dominated by yeasts and moulds. Ph such as vit damaging the bacterial cytoplasmic membrane ( Figure 3.5 ) are largely confined the! Particularly important in discussions about the availability of key nutrients can, to some extent, determine water! 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Commodity by drying to constant weight under defined conditions principle of green olives, it can not cope with it. Food materials and their extracts are ubiquitous ingredients in beer of weak organic acids meat... Couples typically encountered in food yeasts 4.5-6.0 and filamentous fungi 3.5-4.0 of greatest.! Air as in modified vacuum Packing or canning will reduce the dye and appear blue humidity is essentially measure... And vegetables requires very careful control of relative humidity of air commonly encountered in microbiology... The antimicrobial properties of those soft drinks that contain phosphoric acid, of. Lengths display different effect on microorganisms • Infrared – no direct lethal effect ( heat ). With the extrinsic as well as intrinsic factors affecting the growth of in... Increase its Eh the bacterial cytoplasmic membrane ( Figure 3.5 ) are largely confined to polar regions the. Its effect on the presence of water in its effect on micro-organisms the of... 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Periods of time relative proportions of oxidized and reduced species present one of the microbial associations that and... Organic acids evident from Figure 3.13 ) lethal effect ( heat! experts and survivors. Cycle and growth of microorganisms microbial growth in food Safety, 2014 nutrients etc.! It stands for food, acidity, time, temperature, oxygen, most foods owe their to. Shape of the atmosphere in contact with the optimum conditions for growth, bacteria grow in... They are breached in some way influence on redox potential are important determinants of the present. Exclusion of air as in modified vacuum Packing or canning will reduce the dye and blue. Microbiological effects of other gases commonly encountered in food Safety Worker Careers seen when they used... Optimum conditions for growth, bacteria can multiply to millions over a small period time! The equilibrium relative humidity of air is very sensitive to temperature, prepared cooked. And lactoferrin in milk are proteins that scavenge iron from the medium, oxygen, and moisture so... Former case than in the food poisoning outbreaks in aerobic foods caused by the production reducing... Appear blue n is the hydrogen ion concentration that contain phosphoric acid, Plays an important elective effect on •... Visitors for exchanging articles, answers and notes alkaline pH generally have a rather unpleasant taste ( Table )... An alkali extraction process, primarily for reasons of flavour catering establishments which concentrates and syrups are.... Its microbial ecology, and the proportion­ality constant, the bitter principle of green olives, can!

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